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Airmar 2018 part 2: Ultrasonic and EM speedo mysteries unraveled, plus new UDST800

Airmar_UDST800_ultrasonic_triducer_for_real_2-2018_cPanbo.jpgAirmar’s newly announced UDST800 Ultrasonic Smart Triducer is real! I’ve held one in my freckly hand and even installed it on Gizmo. The reality of a new product would not normally be news, but the history of Airmar’s high tech ultrasonic and electromagnetic speed sensors has frustrated boaters wanting the promised precise and reliable Speed through the Water results, which should be many boaters, I think…

Airmar_DST900-EM_and_ultrasonic_ST900_never_shipped_cPanbo.jpgTo be brutally frank, neither of the Airmar products described above ever materialized as presented, and the same is true for the Ultrasonic DST900 that Kees Verruijt enthusiastically photographed and explained in 2012. In fact, as he mentions, this odd situation started years earlier.

In Oct. 2009 Dan Corcoran’s great review of the Airmar CS4500 Ultrasonic speedo kicked off a discussion about how the high value of accurate STW (sometimes just called Speed, but definitely different from the Speed over Ground delivered by GPS). Sailors crave it for precise calculation of True Wind relative to water, but so do power cruisers for better miles-per-gallon fuel calculations. Accurate STW is also key to meaningful set and drift calculations.

There’s also talk in an old entry about how much better the ultrasonic technology would be as a “smart” transducer able to send data directly to NMEA 2000 networks (or to NMEA 0183). And one Panbo commenter came up with a still-live link to the Airmar DST900-EM — “Available Q3 2009” — while another pasted in an Airmar tech support note about an ultrasonic NMEA 2000 triducer about to come out as the DST900-CL (and those revelations led to a discussion about how electromagnetic speed sensing might compare to ultrasonic).

So in 2009, and again in 2012, there seemed to be at least a hard core crowd ready and willing to spend extra money to say goodbye to their paddlewheel speed sensor in favor of more accurate and reliable technology. They never got a chance to pull out their wallets. What happened?

I do not know why Airmar made several false starts in this area — there’s no upside I can think of — but I can theorize. As mentioned in my Part 1 entry about Airmar’s certified installer program, the company mostly works with OEM’s, not retail customers, and maybe trial balloons are more normal in that environment. It’s also an engineer-run company, so maybe testing trumped marketing. Perhaps, above all, the market for premium speed sensors is small, while Airmar has been relentlessly pushing out into other markets while also driving the CHIRP fishfinder revolution.

And none of this matters much now, except to hopefully clear the air and set the stage before we discuss Airmar ultrasonic and electromagnetic speed technology that really, really does exist.

DX900+ MultiLog

Airmar_DX900+_Sail_Pittman_aPanbo.jpgFirst, let’s consider the Airmar DX900+ MultiLog that seems related to the DX900-EM of 2009, but with the intriguing added abilities of measuring leeway and connecting via Bluetooth to apps (in addition to N2K or 0183 cabled connections). Kees broke the story at METS 2016, and I was part of the SAIL team to award it a Pittman.

AirmarCast_app_screens_with_DX900+_aPanbo.jpgGiven the possibly embarrassing history, I was a little surprised that the DX900+ took a while to ship, but it is now available at outlets like iMarine (and the slick looking iOS AirmarCAST app above is available for download). In fact, occasional Panbo contributor Jeremy Anwyl bought one last summer for his latest vessel — a mighty Nordhavn 76 — and I was hoping to host a dual review with Kees reporting on his DX900+ beta testing experience aboard the awesome sloop Merrimac.

Unfortunately, neither Jeremy nor Kees have reliably working DX900+ sensors yet. It’s not worth getting deep into the details here as both cases are still being investigated by Airmar tech support, but it may be that EM is best suited to simple 12v electronic systems, or maybe that the NMEA 0183 option is preferable because then the power supply can be kept especially clean.

Also, Airmar tells me that numerous DX900+ are out in the field without reported problems, so here’s hoping an owner of this highly innovative sensor will report in about actual results. And that Jeremy and/or Kees will be able to report on working EM sensors soon.


So the ST900/950 ultrasonic speed sensor Kees also reported on at METS 2016 got my “Close but no… cigar” markup mockery because it didn’t actually ship until after Airmar decided that UST800 Smart Sensor and UST850 Smart Sensor titles made more sense. {Correction 2/20: some ST900/950 sensors did ship before the name change.}

I agree that UST800 is a good designation for the ultrasonic version of the ST800, but the best news is that the two UST sensors — the only difference is which standard Airmar casings they fit — are really available. The online prices starting at $918 are not trivial — Airmar does not list retail pricing — but consider this performance graph.


If I understand this graph correctly it shows a boat alternately accelerating and decelerating over about 18 minutes while speed is measured by paddlewheel, ultrasonic, and GPS. The paddlewheel fails quite badly over about 35 knots, but if you click the graph bigger and look closely, you’ll see that the paddlewheel isn’t doing very well at low speeds either.

Moreover, I think it’s safe to presume that Airmar used a clean ST800 or similar paddlewheel sensor for this test, and paddlewheel performance most definitely changes with fouling. Also, note the latency. In acceleration the Ultrasonic usually measures speeds slightly higher than the GPS and paddlewheel and vice versa as the boat slows down, which certainly suggests better responsiveness. But I’d like to see for myself…

Brand new UDST800!


When I asked about testing a UST sensor in January, I was delighted to learn that a UDST would not only be announced soon and was already in production. And, lo, there it is, posing in my shop before I took it down to the boat (and quite like the DST900 Kees saw in 2012).

The UDST800 ultrasonic triducer was also announced early this week — press release PDF here — and though it’s not yet listed at Airmar, here is the combined UDST/UST install guide PDF.

It will be awhile before I actually get to see UDST underway performance in Gizmo, but you can see on the Simrad NSS evo2 screens below that it’s real and ready to go.


Similar Posts:

Maretron ultrasonic wind sensor, the plot thickens
November 14, 2005

Airmar CS4500 Part I (Reliable Boat Speed)
October 5, 2009

Airmar Smart Sensors, lots
September 21, 2005

Ultrasonic Weather Wars, w/ Raymarine as Switzerland
June 23, 2006

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher since 4/12/2005, and now excited to have Ben Stein as a very able colleague. Panbo is going to the next level in 2018 and beyond.

25 Responses

  1. Henning says:

    I might want to retire my CS4500, in use since 2012 and still in working order, but first I would have to see some satisfied user reports. So I’ll be very interested how a UDST 800 works on Gizmo.
    My CS4500 has quite severe problems with silt in the water like you find in shallow waters with lots of tidal activity (such as the Waddenzee). In these areas, I regularly have crazy readings that swing, in sort of a sinusiodal curve, between about 0.5 knots and 9 knots while going at a steady 6 knots, confirmed by GPS. Each full curve is maybe 20 seconds long. A minute later, with maybe a little less silt in the water, all is back to normal until it starts again a few minutes later. This never happens in clearer water (e.g. not once in 2.000nm of Baltic sailing of last season).
    Another question I have is how fine you can calibrate it. An absolute must for me is ability to apply correction factors for multiple speed levels. In addition to that I would highly appreciate ability to apply different correction factors depending on roll of the boat (port bow or starboard bow). My boat or my install location definitely needs this – and it’s a standard boat and standard install location.
    In theory, the roll-dependent correction should be no problem as there is continuous supply of roll (and tilt) via NMEA 2000 from my H2183 compass.
    The last point, in my opinion, is critical for a sensor like this. It’s targeted at those that badly want precise speed. But the proposition “buy this sensor and you will have breathtakingly accurate speed” is, unfortunately, naive. You need to invest a lot of time in calibration, and, to be able to do that, you need software that allows very fine corrections in a number of different dimensions.
    Otherwise it’s just a sensor that never fouls – also good, of course.

  2. Dan Corcoran (b393capt) says:

    Great Entry, especially the history, it has been a long journey of product announcements. The ability to measure leeway, N2K, and bluetooth access to the sensor being particularly appealing features.
    Over the years it has seemed like two different technologies were being developed by Airmar, the “EM” for example being different from how the CS4500 senses speed through water as described in a detailed product brochure.
    Ben, does the UDST800 and DX900+ have the same guts/technology? Are they the same or different from the CS4500? Can the UDST800 measure leeway and accessed from bluetooth?

  3. No, Dan, just the DX900+ can measure leeway speed/angle and also roll/pitch, as seen in AirmarCAST screens above. It’s also the only transducer with Bluetooth.

  4. Henning, according to the UDST/UST intall guide linked to above, speed calibration is possible, but it’s vague beyond that, dependent apparently on what display you’re using in conjunction. And of course some systems can apply calibration independent of the transducer (but then it only shows up within that system, not to other N2K displays).
    At any rate, this is definitely an interesting and still largely unknown aspect of these speedos. I did check Gizmo’s Maretron DSM to see if it could calibrate the UDST800 like it could the Maretron DST110 (equivalent to Airmar DST800). No dice, but maybe coming in Maretron update?
    Also, I don’t buy that every boat speedo install needs multipoint calibration. I saw good results with Maretron’s calibration, though it did need to be adjusted as paddlewheel got dirty.
    Plus we have not heard about real world results from Airmar’s latest ultrasonic tech. It may look a lot like what Kees saw in 2012 but I gather they’ve working on the Echo Correlation Processor:
    “The UST800/UST850’s echo correlation processor has patented adaptive digital signal processing that automatically adjusts according to boat speed, water depth and water clarity for most accurate speed readup. Reliable operation in both salt and fresh water.”

  5. Francisco says:

    Hi I have tried a DX900 plus plus in my carbon cruiser racer. I had great excitement for it, measuring leeway is a killer app. Unfortunately reality has not met expectations. Speed readings are all over the place. To check power supply issues, I disconnected all electronics and fed it directly from the batteries , reading speed via Bluetooth. Insert far from a marina to limit electromagnetic noise. It does not work I am sorry to report. Can be the carbon hull? Don’t know. Francisco

  6. Thanks for the report, Francisco. An Airmar representative tells me that he thinks “the culprit in this case is the carbon hull” and wonders if you have worked with Airmar tech support?

  7. Crystal Blues Digital says:

    Raymarine sold the CS4500 for a while as a transducer option. We bought one gladly, being fed up with cleaning the paddle wheel transducer. It was installed back in 2003 and is still working, though the temperature probe failed last year. And yes, it wasn’t initially reliable in muddy water, though it always did seem to adapt with time – the Rajand River in Borneo was our haunt for some years. The Raymarine guys tell me they had a bit of trouble with these, and that we’re a lucky boat. So be it. Now I’m hoping for a DX900 for Christmas.

  8. Francisco says:

    Ben. Developing a new electromagnetic sensor, aimed at the racing market, which is incompatible with carbon hulls, seems surprising. I will call airmar support in a few days when I have a bit of time. But the truth is that I have tried it In several conditions, making sure that the level of a electromagnetic noise was low. I was also quite careful to check that the unit was properly inserted in the sleeve. Unless I am proven wrong, I don’t think this product is up to standard for the general public. This may also explain why it was in the making for such a long time after the announcement. Regards

  9. Nick says:

    The Airmar situation over the last decade has been quite surprising, given we had very good and reliable sonic speed devices in 1997.

  10. Ian says:

    Interesting discussion on this subject on Sailing Anarchy:
    from about post #63.
    Maybe the UDST800 isn’t so hot and paddlewheels’ days aren’t over yet?

  11. Compass Marine says:

    What a coincidence that I see this article now. I just got off the phone with tech support regarding a customers failed CS4500 ultrasonic speed unit. It was not a very fulfilling call nor all that reassuring for me to continue installing these.
    What I learned.
    #1 There is no field test for the transducer to confirm whether it is the problem or not.
    #2 There is no repair of the transducer, if it is what failed.
    #3 Other than confirming 12V power at the control box there is no field test to confirm the control box is working.
    #4 There is also most likely no repair to the circuit board (they were not too committal in this) of the CS4500 control box.
    So, I can send it in $$ and they can possibly test it and tell me which part is bad, the transducer or control box, but then they could not confirm availability of parts for this unit. I even gave them the model revision number; 18-449-01 Rev.01
    I have already confirmed it is not the display with a test unit I keep around.
    These ultrasonic sensor kits are very expensive and this one failed for no apparent reason. It’s mounted dry, has remained bone dry, zero corrosion etc. and I have no way to test it nor to even get it repaired. hard to tell a customer whose vintage Data Marine stuff that lasted 31 years that his few year old very expensive Airmar ultrasonic speed unit is dead.
    Obviously I will need to replace it but with what? These new units are not yet proven. If I go to a new unit it will mean a new display too $$$$ as the new ultrasonic units apparently do not send an analog pulse like the CS4500 could..
    Frustrating to say the least.

  12. Always good to hear from you, RC, though what you describe is depressing. I’m hopeful that that “new” UDST/UST transducers and processing boxes are well developed at this point, but some aspects remain mysterious and only time will indicate their reliability.
    For instance, it’s still unclear to me what sort of calibration is possible, and what devices will be able to do it. The Maretron DST110 (sourced from Airmar) that I pulled to try out the UDST800 could be calibrated against SOG and even though it uses a paddlewheel I was able to get a good match between STW and SOG in zero current conditions, and hence fairly good Set/Drift calculations.
    Also I noticed that the Raymarine i70s appears to have multi-point speed calibration for smart speed sensors, as shown in this entry:

  13. I just updated the DX900+ section of this entry to reflect the fact that Jeremy Anwyl’s had been bottom painted, which definitely affects performance and is therefore prohibited in the installation instructions. He will be testing a fresh DX and also a UDST800 (which can be painted).

  14. Jonathan U says:

    Just sent a DX900+ to a client for a new BB&G H5000 install. And as we’ve done on every system for the past decade that requested a sonic sensor, suggested they install an analog paddlehweel prewired into the system for rapid swap if (“when” might be more accurate) the sonic unit fails. I’m optimistic for the DX but history indicates having a backup readily in place is a good move.

  15. Jeremy says:

    Not to add to any confusion, but the bottom paint I applied to the DX900+ is water based and was allowed. (At least according to the owner’s guide I have.) I think Airmar may be updating the manual to state that no bottom paint of any kind should be used, but not sure about this.
    As for the UDST800, I tried one this morning and in brief sea trials can say it looks very encouraging.

  16. LarryR says:

    Hi All, We have two DX900+ units on our 60′ cat and we will be testing them against the ST200’s also installed over the next two regattas in the Caribbean in the next 3 weeks. Any tips you can give me about what to look for. My plan is to use the ST200s into our H5000 to start with and log the data from N2K into Expedition at the same time and compare. I have not calibrated the 900s as yet, in case you have any tips.
    Larry Rosenfeld

  17. peter says:

    Ben, do you know if UDST is (like DX900) dual-axis speed-to-water system or not. European dealer is not able to confirm this.

  18. Sorry but that’s a no, Peter. The UDST outputs the standard set of Depth, Speed through Water, and water Temperature values. By contrast, the DX900 also measures transverse speed, calculates leeway direction, and more. That’s why it sort of needs the Bluetooth app seen up in the entry, at least until some marine instruments learn to display those values:


    • peter says:

      Thank you Ben for information.

      I have noticed PGN 128000 issue already. Also dual-axis STW seems to be a problem for N2K-based MFDs.

  19. Has anyone actually gotten their hands on the UDST? My Airmar DST110 paddlewheel has stopped working again, likely due to junk growing on it, and I would love to know if the UDST actually is something a sailboat should/could use.

    • Keefer Douglas says:

      Seconded! Doesn’t appear to be for sale yet at any of the usual suspects, and Google just returns a link to this article and not much else…

      • The word from Airmar: “​As far as the UDST800, we have been shipping some but they already have a home when we ship them. UDST production is ramping up to a higher level in the next two weeks. ”

        Also, if you have an installer/dealer with a Gemeco account, he or she can probably get one faster than you’ll see them show up in retail channels.

  20. Anyone know if the UDST800 can be mounted inside a fibreglass hull as you could do with many depth sounders? (temperature may be off, but I would expect depth to work… not sure about speed…)

  21. Eliboat says:

    I installed a dx900 in the boat this year. The first one would not transmit depth or temp over the n2k network, but using the app I could see that the transducer was measuring these items, along with leeway, heel, trim etc. Airmar sent a new one to me and everything was transmitted via the network. The problem now is that my speed readings are wildly inaccurate. Yesterday I was tied to a dock on the Piscataqua river and it was reading the current from .8kts up to 5kts. Once underway against the current for the trip home, the Speedo cycled from under a knot to over 9kts. Not only was this clearly wrong, but as my friend who was with me pointed out, had the water speed been changing in that fashion, as quickly as it was, we wouldn’t have been able to stand on the deck without holding on tight to something.
    Not sure what the issue is yet, as I haven’t had a chance to call Airmar.

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